1. Field of the Invention
The present invention relates to electronic circuitry using a multilayer circuit board that includes a buried capacitor.
2. Description of the Prior Art
In accompaniment with the downsizing of portable equipment, a technology for forming a passive component such as a capacitor within the board in order to enhance the mounting density has become more prevalent, especially in cellular phones. The technology that is in the mainstream at present employs a ceramic board. Attention, however, is now focused on a resin-based board with passive elements built in, because such a board is light-weight, not subject to cracking, inexpensive, etc. When forming a capacitor within a board, employing a thin film capacitor whose capacitive density can be made higher is advantageous in downsizing the board. U.S. Pat. No. 5,027,253 has disclosed a conventional technology concerning a board in which the thin film capacitor is buried. This technology will be explained below, using FIG. 4. The board includes a first signal core 121 including a first thin film copper wiring 123 that has a first thin film copper electrode 125, a second signal core 131 including a second thin film copper wiring 123 that has a second thin film copper electrode 135 which overlays the first thin film copper electrode, and an epitaxial thin film 151 of a dielectric material positioned between the first thin film copper electrode and the second thin film copper electrode. Here, the first thin film copper electrode, the second thin film copper electrode, and the epitaxial thin film of the dielectric material form the integrally-buried type thin film capacitor 141 inside the multilayer circuit package 101. The first signal core and the second signal core are formed of a metal having a high electrical-conductivity, e.g., copper, silver, and aluminum. The epitaxial thin film of the dielectric material between the first thin film copper electrode and the second thin film copper electrode is formed of a sputtered film of ceramics, the representatives of which are, e.g., calcium titanate, barium titanate, aluminum oxide, beryllium oxide, and aluminum nitride. The formation of the structure like this makes it possible to implement, inside the package, the capacitor including the electrodes and the dielectric. Here, the electrodes have the electrical-conductivity higher than those of metals, e.g., Y, Ti, Zr, Ta, Hf, Nb, Mo, and W, and the dielectric has a dielectric-constant higher than those of oxides of, e.g., Y, Ti, Zr, Ta, Hf, Nb, Mo, and W. This implementation allows a capacitor element to be removed from the package surface, thereby enlarging a package surface available for a logic circuit chip/module and thus enhancing the package density.
In the conventional multilayer circuit package that U.S. Pat. No. 5,027,253 has proposed, there existed a problem that it is difficult to cause a ceramic, i.e., the dielectric material, to grow epitaxially on the first signal core. This is because the lattice constant of the metal such as copper, silver, and aluminum does not coincide with the lattice constant of the ceramic planned to grow on the metal. As a result, the sputtered thin film of the ceramic became more likely to exhibit an amorphous growth instead of the crystallization growth. The resultant amorphous thin film of the ceramic exhibits a lower dielectric-constant as compared with an epitaxial thin film that has grown by crystallization growth. Accordingly, the capacity value of a capacitor fabricated using this amorphous thin film becomes smaller than that of the capacitor fabricated using the epitaxial thin film.
Moreover, when employing the easily oxidized metal such as copper or aluminum as the first signal core, sputtering the oxide ceramic thereon resulted in the following problem or the like: The sputtering oxidizes the surface of the metal that forms the first signal core, thereby forming an oxide layer other than the ceramic thin film. This increases the thickness of the dielectric layer, thus lowering the capacity value.
Also, when the capacitor is built inside the multilayer circuit board, no limitation is imposed on the layout in order to enhance the mounting density. This condition makes it likely that a three-dimensional intersection will occur between the built-in capacitor and a signal line formed in a layer other than the layer where the capacitor has been formed. If the capacitor and the signal line intersect to each other, a signal interference occurs therebetween, thereby resulting in a problem that a deterioration occurs in the performance of the electronic circuit using this multilayer circuit board. In the conventional multilayer circuit package, no consideration has been given to the signal interference between the capacitor formed inside the multilayer circuit board and the signal line formed in the layer other than the layer where the capacitor has been formed.
Although there exist capacitors of a variety of uses, e.g., an impedance-matching capacitor and a bypass capacitor, as capacitors used in an electronic circuit, the performances required for the respective uses differ from each other. In order to downsize further the electronic circuit using the multilayer circuit board, it is required to build as many capacitors as possible inside the multilayer circuit board independently of the uses. However, if precision capacitors, e.g., impedance-matching capacitors, are built inside the multilayer circuit board, manufacturing variations in the capacity values become a serious problem. In the conventional multilayer circuit package, no consideration has been given to precision capacitors.
It is an object of the present invention to provide an electronic circuit equipment using a multi-layer circuit board that includes a built-in thin film capacitor having a high dielectric-constant thin film dielectric.
The above-described object of the present invention can be accomplished by the following method: In the electronic circuit equipment using the multi-layer circuit board on which a semiconductor chip is mounted, the thin film capacitor is provided on the multilayer circuit board. Next, a first electrode of the thin film capacitor and a first wiring of the multilayer circuit board are electrically connected to each other, and a second electrode of the thin film capacitor and a second wiring of the multilayer circuit board are electrically connected to each other, respectively. Finally, the thin film dielectric of the thin film capacitor is caused to grow epitaxially with the first electrode as its ground.
The employment of the method like this makes it possible to use, as the first electrode, a metal on the top portion of which the thin film dielectric can be grown epitaxially. The employment thereof also allows a high electrical-conductivity metal to be used as the first wiring conductor. As a result, it becomes possible to form, inside the multilayer circuit board, the thin film capacitor including the high dielectric-constant thin film dielectric.
Here, the following configuration is employed: Of the electrical connection between the first electrode and the first wiring and the electrical connection between the second electrode and the second wiring, at least one of the connections is established via a hole (hereinafter, referred to as xe2x80x9cvia-holexe2x80x9d or xe2x80x9cthrough-holexe2x80x9d) bored in a resin forming the multilayer circuit board, thereby performing the transmission/reception of a signal between the thin film capacitor and a wiring provided in another layer. This configuration allows the thin film capacitor to be formed in every layer inside the multilayer circuit board, thus making it possible to increase the degree of freedom in the board design.
Different conductors are deposited so as to form the first electrode and the first wiring into one and the same pattern. This formation allows the electrode and the wiring pattern to be formed without damaging the above-described features and using a resist pattern formed by the same mask or film, which makes it possible to reduce the board manufacturing cost.
The second electrode is formed so that the area thereof is narrower than that of the thin film dielectric and the second electrode is positioned on the inner side of the thin film dielectric. This formation makes it possible to prevent the thin film capacitor from getting into a malfunction as a capacitor, thereby enhancing the reliability. Here, this malfunction occurs when the first electrode and the second electrode are electrically connected via an impurity adhering to an outer-circumference side surface of the thin film dielectric.
The coating of the thin film dielectric material can be performed to prevent the electrical short-circuit between the first electrode and the second electrode. This coating is effective on the manufacturing.
As the metal on the top portion of which the thin film dielectric can be grown epitaxially, it is preferable to use a metal selected from a group consisting of Ru, Pt, and Pd.
As the thin film dielectric that grows epitaxially on the metal selected from the group consisting of Ru, Pt, and Pd and that exhibits the high dielectric-constant, it is preferable to use a ceramic thin film of an oxide, the representative of which is strontium titanate, or a nitride.
A metal selected from a group consisting of Cu, Au, Ag and Al that have a small conductive loss is preferable as the first electrode and the second electrode.
Also, the first electrode is formed into a double-layer conductor layer having a first connection layer that is positioned on a plane of the first electrode opposite to the thin film dielectric and that is formed of a metal different from the metal of the first electrode. This formation increases the adhesion between the first electrode and the resin or the first wiring conductor bonded onto the plane of the first electrode opposite to the thin film dielectric, thereby being capable of enhancing the reliability. A metal selected from a group consisting of Cr, Mo, and Ti is preferable as the first connection layer as described above.
The second electrode is formed into a double-layer conductor layer having a second connection layer positioned on a plane facing the thin film dielectric and formed of a metal different from the metal of the second electrode. This formation increases the contact characteristics between the second electrode and the thin film dielectric, thereby being capable of enhancing the reliability. A metal selected from the group consisting of Cr, Mo, and Ti is preferable as the second connection layer as described above.
Also, of the first electrode and the second electrode, an electrode that is positioned nearer to a transmission-line conductor formed on the multilayer circuit board is set at the grounding potential. This setting makes it possible to reduce a signal interference between the transmission line and the electrode that has not been set at the grounding potential within the thin film capacitor having a capacity value with respect to the grounding potential. Consequently, no deterioration occurs in the performance of the electronic circuit equipment.